Cerberus

Cerberus is a Titan from Greek and Roman mythology. A three-headed anthropomorphic canine, Cerberus was a fierce and very strong hellhound who guarded Erebus, preventing the dead from escaping the underworld and the living from entering it.

History
Tasked with overseeing the defense of Erebus from both the living and dead, Cerberus was a nearly-invincible monster. King Eurystheus of Tiryns tasked Heracles with capturing Cerberus as the final of his labors, in recompense for the killing of his children while driven insane by Hera. Heracles rescued Theseus and overpowered Cerberus, whom he slung over his back and dragged to Eurystheus. He was brought to a secret grove of Demeter, and Cerberus later escaped, returning to the underworld to guard the dead.

At times, Cerberus was released from the underworld by Greeks with the titan gate to be used in battle against their opponents. As a very strong creature, Cerberus was very difficult to defeat in battle.